A conventional internal combustion engine, such as a four stroke cycle engine typically used in passenger vehicles, uses an exhaust gas recirculation system for reducing emissions of oxides of nitrogen. In such systems, the exhaust gas is recirculated back to the combustion chamber thus reducing peak combustion temperature. Because formation of oxides of nitrogen increases as combustion temperature increases, the presence of exhaust gas in the combustion chamber reduces oxides of nitrogen.
It is known design practice to obtain exhaust gas recirculation by altering the valve timing normally associated with a four stroke OTTO cycle internal combustion engine. Internal exhaust gas recirculation will result from the overlap in the operation of the intake valve and the operation of the exhaust valve (i.e., the intake valve is open while the exhaust valve has not yet closed). As a result, residual gases are reinducted into the cylinder along with a fresh air/fuel mixture.
The use of an internal exhaust gas recirculation system of this kind has limitations since excess overlap precludes stable engine operation at light loads, which is characterized by misfire and increases in HC emissions.
It is known design practice also to use an external EGR system, rather than an internal EGR system to meet limit values for oxides of nitrogen. Such systems return engine exhaust gas to the fresh air/fuel mixture.
A recently developed internal exhaust gas recirculation system for four stroke cycle, spark-ignition, internal combustion engines involves a dual equal variable camshaft timing strategy (DE/VCT). This strategy includes phase shifting of the intake and exhaust valve timing relative to the crankshaft position as a function of engine operating variables. This is done principally to improve fuel economy and emissions at part load, which makes it possible to eliminate the external EGR system.
The DE/VCT strategy for phase shifting the intake and exhaust events involves phase shifting equally the opening of the intake valve and the closing of the exhaust valve into the intake stroke phase of the engine combustion cycle. Likewise, the exhaust valve opening is delayed so that this event occurs closer to the bottom dead-center position of the piston, and the intake valve closing occurs farther from the bottom dead-center position due to the retarded overlap of the intake valve and exhaust valve functions.
At the beginning of the intake stroke, the DE/VCT strategy causes the intake valve to be closed while the exhaust valve is open. Exhaust gases then are drawn into the cylinder from the exhaust port during the intake stroke, which results in reduced oxides of nitrogen in the exhaust gases and reduced unburned hydrocarbons. Pumping losses associated with the intake stroke are reduced using the DE/VCT strategy because the exhaust gas drawn into the cylinder is at exhaust back pressure. As the intake valve begins to open, gases in the cylinder are expanded since flow through the exhaust port is not sufficient to maintain exhaust gas pressure. This results in higher cylinder pressure during the first part of the intake stroke, which reduces pumping work. Further, with the increased residual gas drawn into the combustion chamber, a higher manifold absolute pressure is required to maintain a given load. This higher manifold absolute pressure also results in reduced intake stroke pumping work.
Camshaft phase shifting associated with the DE/VCT strategy also causes a later intake valve closing, causing more of the fresh charge to enter the intake port during the first part of the compression stroke. This too requires a higher manifold absolute pressure to maintain a given load. The intake stroke pumping work is reduced for this additional reason.
Although a late intake valve closing has been determined to reduce pumping losses, it also reduces the effect of compression ratio and temperature near the end of the compression stroke, thus reducing burn rate and dilution capability, which limits the fuel economy benefit. This disadvantage, however, is overcome by the DE/VCT phase shifting strategy because of the higher initial temperature of the residual gas and fresh charge mixture. This higher initial temperature results from the use of internal exhaust gas recirculation rather than external exhaust gas recirculation. The disadvantage of using higher initial gas temperature, which tends to promote detonation, is offset by the reduction in effective compression ratio due to the late intake valve closing. Further, the use of a late exhaust valve opening results in increased expansion work, which results in improved efficiency (i.e., indicated specific fuel consumption).
It is has been observed that at higher speeds and loads, the use of the dual equal variable camshaft timing strategy without an external exhaust gas recirculation system results in exhaust stroke pumping losses because of the late opening of the exhaust valve. Furthermore, during the intake stroke, the low valve lift at high piston speeds, resulting from the later opening of the intake valve, increases intake stroke pumping losses.